Electrolytic capacitor

ABSTRACT

A disclosed electrolytic capacitor includes a capacitor element. The capacitor element includes an anode body, a dielectric layer formed on a surface of the anode body, a cathode body, and an electrolyte layer and a separator that are disposed between the dielectric layer and the cathode body. The electrolyte layer includes a non-aqueous solvent and conductive particles. A ratio D/T of an average maximum diameter D of the conductive particles to an average thickness T of the separator is in a range of 0.01 to 0.9.

TECHNICAL FIELD

The present disclosure relates to an electrolytic capacitor.

BACKGROUND ART

Capacitors for use in electronic devices are required to have a highcapacity and a low value of an equivalent series resistance (ESR) in ahigh frequency range. If ESR is large, various problems occur. Forexample, when a ripple current flows, the amount of heat proportional tothe ESR is generated, and the generated heat causes a reduction in thecharacteristics of the conductive polymer.

One promising high-capacity and low-ESR capacitor is an electrolyticcapacitor that uses a conductive polymer such as polypyrrole,polythiophene, polyfuran, and polyaniline. PTL 1 (WO 2012/117994)discloses, as a conductive polymer solution for forming a solidelectrolyte layer, “A conductive polymer solution, comprising aconductive polymer, a polysulfonic acid or a salt thereof whichfunctions as a dopant to the conductive polymer, a mixture of a polyacidand a carbon material, and a solvent” (Claim 1 of PTL 1). In addition,PTL 1 discloses a solid electrolytic capacitor manufactured using theconductive polymer solution.

CITATION LIST Patent Literature

-   [PTL 1] WO 2012/117994

SUMMARY OF INVENTION Technical Problem

Currently, there is a need for an electrolytic capacitor having a lowrate of increase in ESR over a long period of time. Under such acircumstance, an object of the present disclosure is to provide anelectrolytic capacitor having a low rate of increase in ESR over a longperiod of time.

Solution to Problem

An aspect of the present disclosure relates to an electrolyticcapacitor. The electrolytic capacitor is an electrolytic capacitorincluding a capacitor element, wherein the capacitor element includes ananode body, a dielectric layer formed on a surface of the anode body, acathode body, and an electrolyte layer and a separator that are disposedbetween the dielectric layer and the cathode body, the electrolyte layerincludes a non-aqueous solvent and conductive particles, and a ratio D/Tof an average maximum diameter D of the conductive particles to anaverage thickness T of the separator is in a range of 0.01 to 0.9.

Advantageous Effects of Invention

According to the present disclosure, it is possible to obtain anelectrolytic capacitor having a low rate of increase in ESR over a longperiod of time.

While the novel features of the invention are set forth in the appendedclaims, the invention, both as to organization and content, will bebetter understood and appreciated, along with other objects and featuresthereof, from the following detailed description taken in conjunctionwith the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an exemplaryelectrolytic capacitor according to the present disclosure.

FIG. 2 is a view schematically showing a part of the electrolyticcapacitor shown in FIG. 1 .

DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the present disclosure will bedescribed by way of examples. However, the present disclosure is notlimited to the examples described below. Although examples of specificnumerical values and materials may be given in the followingdescription, other numerical values and materials may be used as long asthe effects of the present disclosure can be achieved. In thisspecification, by “from numerical value A to numerical value B”, it ismeant that the range includes the numerical value A and the numericalvalue B.

(Electrolytic Capacitor)

An electrolytic capacitor according to the present embodiment includes acapacitor element. The capacitor element includes an anode body, adielectric layer formed on a surface of the anode body, a cathode body,and an electrolyte layer and a separator that are disposed between thedielectric layer and the cathode body. The electrolyte layer includes anon-aqueous solvent and conductive particles. A ratio D/T of an averagemaximum diameter D (μm) of the conductive particles to an averagethickness T (μm) of the separator is in a range of 0.01 to 0.9.

The conductive particles undergo substantially no degradation due toheat. For this reason, by adding the conductive particles to theelectrolyte layer, it is possible to suppress an increase in ESR over along period of time. By setting the ratio D/T to 0.9 or less, a shortcircuit associated with the conductive particles can be suppressed. Bysetting the ratio D/T to 0.01 or more, ESR can be particularly reduced.

The average thickness T of the separator can be measured using aconstant-pressure thickness measuring instrument compliant with JIS P8118.

The average maximum diameter D of the conductive particles can bemeasured by the following method. First, an image of the interior of theelectrolyte layer or the top of the separator is captured using a device(an optical microscope, an electron microscope, etc.) capable ofperforming image capturing at a magnification of greater than or equalto 100× and identifying the shapes of particles. The maximum diameter ofthe particles is measure by subjecting the resultant image to imageprocessing. The maximum diameter is measured for 10 arbitrarily selectedparticles, and the arithmetic average thereof is determined as theaverage maximum diameter D. Unless there is any particular problem, theaverage maximum diameter D is measured using this method. As anothermethod, the volume-based particle size distribution may be measuredusing sieving, light scattering, or precipitation, and a D₉₀ diameter atwhich a cumulative volume is 90% may be determined as the averagemaximum diameter D.

The electrolytic capacitor according to the present embodiment maysatisfy the following condition (1), and may further satisfy theconditions (2) and/or (3). By satisfying the conditions (1), (2), and(3), an increase in ESR can be particularly suppressed over a longperiod of time.

-   -   (1) The above-described ratio D/T is in the range of 0.01 to        0.9. The ratio D/T may be in the range of 0.01 to 0.5 (e.g., the        range of 0.01 to 0.33). When the aspect ratio of the conductive        particles is 3 or more, the ratio D/T may be in the range of 0.1        to 1.0.    -   (2) The average thickness T of the separator is in the range of        10 μm to 200 μm (e.g., the range of 20 μm to 100 μm).    -   (3) The average maximum diameter D of the conductive particles        is in the range of 0.1 μm to 180 μm (e.g., the range of 0.2 μm        to 90 μm).

The capacitor element may include a foil-shaped anode body having adielectric layer on a surface thereof, a foil-shaped cathode body, and aseparator and an electrolyte layer that are disposed between the anodebody (more specifically, the dielectric layer) and the cathode body. Thecapacitor element may be a wound capacitor element or a stackedcapacitor element. In an exemplary wound capacitor element, thefoil-shaped anode body, the foil-shaped cathode body, and the separatorare wound such that the separator is disposed between the anode body andthe cathode body. In an exemplary stacked capacitor element, thefoil-shaped anode body, the foil-shaped cathode body, and the separatorare folded in a zigzag fashion such that the separator is disposedbetween the anode body and the cathode body.

The electrolytic capacitor according to the present embodiment maysatisfy the following condition (4), and may further satisfy thecondition (5). By using these configurations, the effects describedbelow can be achieved.

-   -   (4) The electrolyte layer further includes a conductive polymer.    -   (5) The electrolyte layer includes a dopant of the conductive        polymer. The dopant may be a polymeric dopant containing an        acidic group. In that case, the electrolyte layer may include an        electrolytic solution including a non-aqueous solvent and a base        component dissolved in the non-aqueous solvent. In that case,        the content of the base component in the electrolytic solution        may be 0.1 mass % or more and 20 mass % or less.

By using the conductive polymer including the dopant, it is possible toincrease the conductivity of the electrolyte layer. Examples of theconductive polymer and the dopant will be described below.

The mass of the conductive particles included in the electrolyte layermay be larger than a total mass of the conductive polymer and the dopantincluded in the electrolyte layer. With this configuration, an increasein ESR can be particularly suppressed. A total content M (mass %) of theconductive polymer and the dopant in the electrolyte layer and a contentN (mass %) of the conductive particles in the electrolyte layer maysatisfy M<N. Also, 0.1<N may be satisfied, or 0.05<M may be satisfied.

A ratio N/M, which is the ratio between the content N of the conductiveparticles and a total content M of the conductive polymer and thedopant, may be in the range of 25/75 to 75/25. By using this range, anelectrolytic capacitor having excellent characteristics can be obtained.

The conductive particles may be conductive inorganic particles. Theconductive particles may be particles of a conductive carbon material.For example, the conductive particles may include at least one selectedfrom the group consisting of particles of carbon black, particles ofcarbon nanotube, particles of graphite, and particles of graphene. Theseparticles are preferable in that their average particle size, thestructure between particles, and the surface properties can becontrolled in various manners. The conductive particles may be composedonly of one type of particles of these particles, or may be composed ofa plurality of types of particles of these particles.

The conductive particles may have a spherical shape or a flaky shape.The type of conductive particles having a flaky shape is notparticularly limited, and the conductive particles may be flakyparticles made of a conductive carbon material. For example, graphiteand graphene can easily take a flaky form, and flaky particles thereofare readily available. By using flaky particles, it is possible toachieve effects such as increasing the conductivity of the electrolytelayer, increasing the affinity between the electrolyte or the separatorand the conductive particles, and suppressing the uneven distribution ofthe conductive particles in the electrolyte and the separator.

The average aspect ratio of the flaky carbon particles may be 2 or more,or 3 or more. The average aspect ratio of the carbon particles can bedetermined in the following manner. First, an image of carbon particlesis obtained using a Scanning Electron Microscope (SEM). In the obtainedSEM image, a plurality of (e.g., 10) carbon particles are arbitrarilyselected. Next, for the selected carbon particles, a maximum diameter D1is measured, and a maximum diameter D2 in a direction orthogonal to themaximum diameter D1 is further measured. For each of the carbonparticles, a ratio D1/D2 of D1 to D2 is determined as an aspect ratio,and the aspect ratios of the carbon particles are arithmeticallyaveraged, whereby an average aspect ratio is determined.

The conductive particles may have a whisker (rod) shape. By usingwhisker-shaped particles, it is possible to achieve effects such asincreasing the conductivity of the electrolyte layer, increasing theaffinity between the electrolyte or the separator and the conductiveparticles, and suppressing the uneven distribution of the conductiveparticles in the electrolyte and the separator.

The type of conductive particles having a whisker shape is notparticularly limited, and the conductive particles may be particles madeof a conductive carbon material, or particles made of another inorganicmaterial. For example, the conductive particles having a whisker shapemay be carbon nanotubes or carbon nanofibers. Alternatively, theconductive particles may be particles obtained by coveringwhisker-shaped inorganic particles (e.g., glass fibers) with aconductive metal (e.g., a conductive alloy).

In a preferred example, the cathode body is a conductive foil, and theconductive particles are metal particles. When the cathode body is aconductive foil (e.g., a metal foil), the contact resistance between theconductive polymer and the cathode body may increase. However, when thecathode body is a conductive foil (e.g., a metal foil) and theconductive particles are metal particles, the contact resistancetherebetween can be reduced. Accordingly, the ESR of the electrolyticcapacitor can be particularly reduced.

Except for portions that are characteristic to the electrolyticcapacitor according to the present disclosure, the constituent membersof the electrolytic capacitor according to the present disclosure arenot particularly limited, and known constituting members may be used.Exemplary constituent members of the electrolytic capacitor according tothe present disclosure will now be described.

(Electrolyte Layer)

The electrolyte layer is disposed between and in contact with thedielectric layer and the cathode body. That is, the region between thedielectric layer and the cathode body constitutes the region of theelectrolyte layer. As described above, the electrolyte layer includesthe non-aqueous solvent and the conductive particles, and may furtherinclude a conductive polymer.

(Conductive Polymer)

The conductive polymer included in the electrolyte layer will now bedescribed. In this specification, a conductive polymer (“doudenseikobunshi” in Japanese) may be read as a conductive polymer (“doudenseiporima” in Japanese).

Examples of the conductive polymer include polypyrrole, polythiophene,polyfuran, polyaniline, polyacetylene, and derivatives thereof. Thederivatives include polymers including polypyrrole, polythiophene,polyfuran, polyaniline, and polyacetylene, respectively, as basicskeletons. For example, the derivatives of polythiophene includepoly(3,4-ethylenedioxythiophene). These conductive polymers may be usedalone, or a plurality of them may be used in combination. The conductivepolymer may be a copolymer of two or more monomers. The weight-averagemolecular weight of the conductive polymer is not particularly limited,and may be in the range of 1000 to 100000, for example. A preferredexample of the conductive polymer is poly(3,4-ethylenedioxythiophene)(PEDOT).

Preferably, the conductive polymer is doped with a dopant. From theviewpoint of suppressing dedoping from the conductive polymer, it ispreferable to use a polymeric dopant as the dopant. Examples of thepolymeric dopant include polyvinylsulfonic acid, polystyrenesulfonicacid, polyallylsulfonic acid, polyacrylicsulfonic acid,polymethacrylsulfonic acid, poly(2-acrylamide-2-methylpropanesulfonate),polyisoprenesulfonic acid, and poly(acrylic acid). These may be usedalone or in combination of two or more. These may be included in theform of a salt in the electrolyte layer. A preferred example of thedopant is polystyrenesulfonic acid (PSS). As in the case of theabove-described example, the conductive polymer and the dopant aretypically separate molecules. However, the conductive polymer may be aself-doping conductive polymer including an atomic group (e.g., asulfonic acid group) that functions as a dopant.

The weight-average molecular weight of the dopant is not particularlylimited. From the viewpoint of facilitating formation of a homogeneouselectrolyte layer, the weight-average molecular weight of the dopant maybe in the range of 1000 to 100000.

In the electrolytic capacitor according to the present disclosure, thedopant may be polystyrenesulfonic acid, and the conductive polymer maybe poly(3,4-ethylenedioxythiophene). That is, the electrolyte layer mayinclude poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonicacid.

(Liquid Component)

The electrolyte layer of the electrolytic capacitor according to thepresent disclosure includes a non-aqueous solvent. The electrolyte layermay include an electrolytic solution (non-aqueous electrolytic solution)including a non-aqueous solvent and a base component dissolved in thenon-aqueous solvent. That is, the electrolyte layer of the electrolyticcapacitor according to the present disclosure may include a liquidcomponent. In the following, the liquid component (non-aqueous solventor electrolytic solution) included in the electrolyte layer may bereferred to as a “liquid component (L)”. In this specification, theliquid component (L) may be a component that is liquid at roomtemperature (25° C.), or a component that is liquid at a temperature atwhich the electrolytic capacitor is used. The electrolytic capacitorhaving an electrolyte layer including the liquid component (L) may becalled a hybrid capacitor.

The non-aqueous solvent included in the electrolyte layer may be anorganic solvent, or an ionic liquid. Examples of the non-aqueous solventinclude polyhydric alcohols such as ethylene glycol and propyleneglycol, cyclic sulfones such as sulfolane (SL), lactones such asγ-butyrolactone (γBL), amides such as N-methylacetamide,N,N-dimethylformamide, and N-methyl-2-pyrrolidone, esters such as methylacetate, carbonate compounds such as propylene carbonate, ethers such as1,4-dioxane, ketones such as methyl ethyl ketone, and formaldehyde.

Also, a polymeric solvent may be used as the non-aqueous solvent.Examples of the polymeric solvent include polyalkylene glycol, aderivative of polyalkylene glycol, and a compound obtained bysubstituting at least one hydroxyl group of polyhydric alcohol withpolyalkylene glycol (including a derivative). Specific examples of thepolymeric solvent include polyethylene glycol (PEG), polyethylene glycolglyceryl ether, polyethylene glycol diglyceryl ether, polyethyleneglycol sorbitol ether, polypropylene glycol, polypropylene glycolglyceryl ether, polypropylene glycol diglyceryl ether, polypropyleneglycol sorbitol ether, and polybutylene glycol. Further examples of thepolymeric solvent include an ethylene glycol-propylene glycol copolymer,an ethylene glycol-butylene glycol copolymer, and a propyleneglycol-butylene glycol copolymer. The non-aqueous solvents may be usedalone, or two or more of them may be used as a mixture.

As described above, the electrolyte layer may include a non-aqueoussolvent, and a base component (base) dissolved in the non-aqueoussolvent. Also, the electrolyte layer may include a non-aqueous solvent,and a base component and/or an acid component (acid) dissolved in thenon-aqueous solvent.

As the acid component, it is possible to use polycarboxylic acid andmonocarboxylic acid. Examples of the polycarboxylic acid includealiphatic polycarboxylic acid ([saturated polycarboxylic acid,including, for example, oxalic acid, malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, 1,6-decane dicarboxylic acid, and 5,6-decane dicarboxylicacid]; [unsaturated polycarboxylic acid, including, for example, maleicacid, fumaric acid, icotanic acid]), aromatic polycarboxylic acid (e.g.,phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid,and pyromellitic acid), and alicyclic polycarboxylic acid (e.g.,cyclohexane-1,2-dicarboxylic acid and cyclohexene-1,2-dicarboxylic acid,etc.)

Examples of the above-described monocarboxylic acid include aliphaticmonocarboxylic acid (with 1 to 30 carbon atoms) ([saturatedmonocarboxylic acid, including, for example, formic acid, acetic acid,propionic acid, butyric acid, isobutyric acid, valeric acid, caproicacid, enanthic acid, caprylic acid, pelargonic acid, lauryl acid,myristic acid, stearic acid, and behenic acid]; [unsaturatedmonocarboxylic acid, including, for example, acrylic acid, methacrylicacid, and oleic acid]), aromatic monocarboxylic acid (e.g., benzoicacid, cinnamic acid, and naphthoic acid), oxy carboxylic acid (e.g.,salicylic acid, mandelic acid, and resorcinol acid).

Among these, maleic acid, phthalic acid, benzoic acid, pyromelliticacid, and resorcinol acid are thermally stable, and can be preferablyused.

An inorganic acid may be used as the acid component. Typical examples ofthe inorganic acid include phosphoric acid, phosphorous acid,hypophosphorous acid, alkyl phosphoric acid ester, boric acid,fluoroboric acid, tetrafluoroboric acid, hexafluorophosphoric acid,benzenesulfonic acid, and naphthalenesulfonic acid. Also, a compositecompound of an organic acid and an inorganic acid may be used as theacid component. Examples of such a composite compound includeborodiglycolic acid, borodioxalic acid, and borodisalicylic acid.

The base component may be a compound having an alkyl-substituted amidinegroup, and may be, for example, an imidazole compound, a benzimidazolecompound, or an alicyclic amidine compound (a pyrimidine compound or animidazoline compound). Specifically, 1,8-diazabicyclo[5,4,0]undecene-7,1,5-diazabicyclo[4,3,0]nonene-5, 1,2-dimethylimidazolinium,1,2,4-trimethylimidazoline, 1-methyl-2-ethylimidazoline,1,4-dimethyl-2-ethylimidazoline, 1-methyl-2-heptylimidazoline,1-methyl-2-(3′heptyl)imidazoline, 1-methyl-2-dodecylimidazoline,1,2-dimethyl-1,4,5,6-tetrahydropyrimidine, 1-methylimidazole, and1-methylbenzimidazole are preferable. By using these compounds, acapacitor that exhibits excellent impedance performance can be obtained.

A quaternary salt of a compound having an alkyl-substituted amidinegroup may be used as the base component. Examples of such a basecomponent include imidazole compounds, benzimidazole compounds,alicyclic amidine compounds (a pyrimidine compound and an imidazolinecompound) that are quaternized with an alkyl group or an arylalkyl groupeach having 1 to 11 carbon atoms. Specifically, it is preferable to use1-methyl-1,8-diazabicyclo[5,4,0]undecene-7,1-methyl-1,5-diazabicyclo[4,3,0]nonene-5, 1,2,3-trimethylimidazolinium,1,2,3,4-tetramethylimidazolinium, 1,2-dimethyl-3-ethyl-imidazolinium,1,3,4-trimethyl-2-ethylimidazolinium,1,3-dimethyl-2-heptylimidazolinium,1,3-dimethyl-2-(3′heptyl)imidazolinium,1,3-dimethyl-2-dodecylimidazolinium,1,2,3-trimethyl-1,4,5,6-tetrahydropyrimidium, 1,3-dimethylimidazolium,1-methyl-3-ethylimidazolium, and 1,3-dimethylbenzimidazolium. By usingthese compounds, a capacitor that exhibits excellent impedanceperformance can be obtained.

Also, tertiary amine may be used as the base component. Examples of thetertiary amine include trialkylamines (e.g., trimethylamine,dimethylethylamine, methyldiethylamine, triethylamine,dimethyl-n-propylamine, dimethylisopropylamine,methylethyl-n-propylamine, methylethylisopropylamine,diethyl-n-propylamine, diethylisopropylamine, tri-n-propylamine,triisopropylamine, tri-n-butylamine, and tri-tert-butylamine) and phenylgroup-containing amines (e.g., dimethylphenylamine,methylethylphenylamine, and diethylphenylamine). In particular,trialkylamines are preferable in that the conductivity of theelectrolyte layer is increased, and it is more preferable that at leastone selected from the group consisting of trimethylamine,dimethylethylamine, methyldiethylamine, and triethylamine is included.Also, a secondary amine such as dialkylamine, a primary amine such asmonoalkylamine, or ammonia may be used as the base component.

The liquid component (L) may contain a salt of the acid component andthe base component. The salt may be an inorganic salt and/or an organicsalt. An organic salt is a salt in which at least one of the anion andthe cation contains an organic material. As the organic salt, it ispossible to use, for example, trimethylamine maleate, triethylamineborodisalicylate, ethyldimethylamine phthalate, mono1,2,3,4-tetramethylimidazolinium phthalate, and mono1,3-dimethyl-2-ethylimidazolinium phthalate.

To suppress dedoping of the dopant, the pH of the liquid component (L)may be less than 7, or 5 or less (e.g., the range of 2 to 4.5).

It is important for an electrolytic capacitor to have low ESR. It ispossible to realize low ESR by using an electrolyte layer including aconductive polymer doped with a dopant. However, the present inventorshave found that, when an electrolyte layer including a conductivepolymer doped with a dopant and a non-aqueous solvent (liquid component(L)) is used, the initial ESR is low, but there is a serious degradationphenomenon in which ESR increases over time. As a result ofinvestigating causes thereof, it has been found that the dopant may bemore likely to be dedoped in the electrolyte layer including the liquidcomponent (L). It is considered that this dedoping causes an increase inESR over time. Therefore, it is important to suppress an increase in ESRover time for an electrolytic capacitor including the liquid component(L), as compared with a solid electrolytic capacitor including a solidelectrolyte that does not include the liquid component (L).

The conductive polymer has high conductivity, and is therefore effectivein reducing ESR. However, the conductivity of the conductive polymer isreduced by degradation over time, thus causing an increase in ESR. Inparticular, when the electrolyte layer includes the liquid component(L), the increase in ESR is significant. On the other hand, it isconsidered that the conductive particles undergo substantially nodegradation in conductivity over time. Therefore, adding the conductiveparticles can suppress an increase in ESR over time.

In the electrolytic capacitor according to the present disclosure, thedopant may be a dopant containing an acidic group, or a polymeric dopantcontaining an acidic group. As a result of investigations, the presentinventors have newly found that, in the case of using a dopantcontaining an acidic group, a significant dedoping may occur with anincrease in pH. Therefore, it is particularly important to suppress anincrease in ESR over time in the case of using a dopant containing anacidic group.

In the electrolytic capacitor according to the present disclosure, thedopant may be a polymeric dopant containing an acidic group, and theelectrolyte layer may include an electrolytic solution including anon-aqueous solvent and a base component dissolved in the non-aqueoussolvent. In this case, the base component is likely to cause dedoping,and it is therefore particularly important to suppress an increase inESR over time. As described above, the electrolytic capacitor accordingto the present disclosure includes the conductive particles, and thuscan suppress an increase in ESR over time.

Examples of the acidic group include a sulfonic acid group and acarboxyl group. The polymeric dopant containing an acidic group is apolymer in which at least some of the constituent units contain anacidic group. Examples of such a polymeric dopant include theabove-described polymeric dopants.

In the electrolytic capacitor according to the present disclosure, theamount of the base component in the electrolytic solution may be 0.1mass % or more and 20 mass % or less. When the amount of the basecomponent is 0.1 mass % or more, it is particularly important to use theconductive particles. When the amount of the base component is 20 mass %or less, the base component can be easily dissolved in the electrolyticsolution.

The content of the liquid component (L) in the electrolyte layer may bein the range of 10 to 99.85 mass % (e.g., the range of 30 to 95 mass %).A total content of the conductive polymer and the dopant in theelectrolyte layer may be in the range of 0.05 to 20 mass % (e.g., therange of 1 to 10 mass %). The content of the conductive particles in theelectrolyte layer may be in the range of 0.1 to 80 mass % (e.g., therange of 1 to 30 mass %). These contents may be outside the rangesdescribed herein as long as the effects of the present disclosure can beachieved.

(Conductive Particles)

The conductive particles included in the electrolyte layer will now bedescribed. The conductive particles are particles made of a conductivematerial. Note that the conductive particles are different from theabove-described conductive polymer. Typically, the conductive particlesare made of a material that is not a polymer.

The conductive particles included in the electrolyte layer may includeonly one type of conductive particles, or may include a plurality oftypes of conductive particles. Each of the conductive particles is aparticle having conductivity, and a conductive material is present atleast on the surface of the particle. The conductive particles may beparticles made of a conductive material. The conductive material may beat least one selected from the group consisting of a metal, a conductivecarbon material, a conductive oxide, and a metal-plated material.Example of the metal include gold, silver, copper, nickel, and tin.Examples of the conductive carbon material include carbon black, carbonnanotubes, graphite, and graphene. Examples of the conductive oxideinclude tin oxide, indium oxide, and zinc oxide. Alternatively, theconductive material may be a conductive nickel-phosphorus (Ni—P)material, a conductive indium-tin (In—Sn) material, a conductivetin-silver (Sn—Ag), or the like. The conductive particles may beparticles obtained by coating (e.g., coating by metal plating) thesurface of glass beads, mica powder, glass fiber, carbon fiber, or thelike with any of the above-described conductive materials. Theconductive particles may be metal particles, particles of a conductivecarbon material, or particles of a conductive oxide.

The average particle size of the conductive particles may be in therange of 0.2 μm to 50 μm (e.g., the range of 2 μm to 20 μm). In thisspecification, the average particle size of particles refers to a mediandiameter (D₅₀) at a cumulative volume of 50% in a volume-based particlesize distribution. The median diameter can be determined using a laserdiffraction/scattering particle size distribution measurement device,for example.

(Anode Body)

As the anode body, a metal foil having a dielectric layer formed on asurface thereof may be used. The metal constituting the metal foil isnot particularly limited. From the viewpoint of ease of forming thedielectric layer, examples of the metal constituting the metal foilinclude valve metals such as aluminum, tantalum, niobium, and titanium,and alloys of valve metals. Preferred examples are aluminum and analuminum alloy. Usually, the surface of the anode body is roughened(made porous). The dielectric layer of the anode body is formed on theporous portion (roughened surface). The electrolyte layer is in contactwith the dielectric layer of the anode body.

The dielectric layer formed on the surface of the anode body can beformed by a known method. For example, the dielectric layer may beformed by oxidizing the surface of a metal foil that will form the anodebody through chemical conversion treatment.

(Cathode Body)

A metal foil may be used for the cathode body. The metal constitutingthe metal foil is not particularly limited. Examples of the metalconstituting the metal foil include valve metals such as aluminum,tantalum, niobium, and titanium, and alloys of valve metals. Preferredexamples are aluminum and an aluminum alloy. The surface of the cathodebody may be provided with a chemically converted film, or may beprovided with a coating of a metal (dissimilar metal) different from themetal constituting the cathode body, or a coating of a nonmetal.Examples of the dissimilar metal and the nonmetal include metals such astitanium and nonmetals such as carbon.

(Separator)

As the separator, it is possible to use a sheet-shaped material that canbe impregnated with an electrolyte. For example, a sheet-shaped materialthat is insulating and can be impregnated with an electrolyte may beused. The separator may be a woven fabric, a non-woven fabric, or aporous film. Examples of the material of the separator includecellulose, polyethylene terephthalate, polybutylene terephthalate,polyphenylene sulfide, vinylon, nylon, aromatic polyamide, polyimide,polyamide imide, polyetherimide, rayon, and glass.

The basis weight of the separator may be in the range of 10 to 50 g/m²(e.g., the range of 10 to 30 g/m²). Here, the basis weight is a valuemeasured in accordance with JIS P 8124.

(Exemplary Manufacturing Method of Electrolytic Capacitor)

An exemplary manufacturing method of the electrolytic capacitoraccording to the present disclosure will now be described. Theelectrolytic capacitor according to the present disclosure may bemanufactured by a method other than the method described below. Notethat the matters that have been described for the electrolytic capacitoraccording to the present disclosure can be applied to the followingmanufacturing method, and therefore redundant descriptions may beomitted. For example, the constituent elements of the capacitor elementhave been described above, and therefore redundant descriptions thereofmay be omitted. Matters that will be described for the followingmanufacturing method may be applied to the electrolytic capacitordescribed above.

A manufacturing method according to the present disclosure includes step(i), step (ii), and step (iii). These steps will now be described. Notethat the following describes an exemplary manufacturing method for acase where the electrolyte layer includes a conductive polymer. When theelectrolyte layer does not include any conductive polymer, theconductive particles may be attached in advance to the separator, or maybe attached to the separator in the capacitor element precursor throughimpregnation treatment. Alternatively, the conductive particles may bedispersed in the non-aqueous solvent in advance, and the capacitorelement precursor may be impregnated with the non-aqueous solvent andthe conductive particles simultaneously.

(Step (i))

The step (i) is a step (i) of preparing a capacitor element precursorincluding an anode body having a dielectric layer on a surface thereof.The step (i) may be a step of forming the capacitor element precursor bya known method.

The step (i) is a step of forming a capacitor element precursorincluding a foil-shaped anode body having a dielectric layer on asurface thereof, a foil-shaped cathode body, and a separator disposedbetween the anode body and the cathode body. In this case, as describedabove, the capacitor element precursor may be a wound capacitor elementprecursor or a stacked capacitor element precursor. That is, thecapacitor element precursor may be a wound body.

(Step (ii))

The step (ii) is a step of forming a polymer layer including aconductive polymer and conductive particles by impregnation treatmentsuch that the polymer layer is adjacent to the dielectric layer. Theconductive polymer may be doped with a dopant. In the following, anexample in which the conductive polymer is doped with a dopant will bedescribed.

The impregnation treatment in the step (ii) may be impregnationtreatment (λ) in which the capacitor element precursor is impregnatedwith a dispersion including a conductive polymer doped with a dopant andconductive particles. For example, the capacitor element precursor canbe impregnated with the dispersion by immersing the capacitor elementprecursor in the dispersion. By removing (drying) the dispersion mediumof the dispersion with which the capacitor element precursor has beenimpregnated, the polymer layer including the conductive polymer dopedwith a dopant and the conductive particles can be disposed so as to beadjacent to the dielectric layer. Note that the impregnation treatment(x) may be performed a plurality of times. In that case, a drying stepof removing the dispersion medium of the impregnated dispersion may beperformed before performing the second and subsequent impregnationtreatments (x).

The dispersion medium of the dispersion is not particularly limited, anda known dispersion medium may be used. For example, as the dispersionmedium, an aqueous liquid containing water may be used, or water may beused.

By adjusting the mass (content) of the conductive polymer and the mass(content) of the conductive particles in the dispersion, it is possibleto adjust the ratio therebetween in the formed electrolyte layer. Forexample, by increasing the mass (content) of the conductive polymer inthe dispersion to be larger than the mass (content) of the conductiveparticles in the dispersion, it is possible to increase the mass of theconductive polymer included in the electrolyte layer to be larger thanthe mass of the conductive particles included in the electrolyte layer.

Note that at least a part of conductive particles may be carried inadvance on the separator used for forming the capacitor elementprecursor in the step (i). For example, a separator with the conductiveparticles carried thereon may be used to form a capacitor elementprecursor, and, in the impregnation treatment (x), the capacitor elementprecursor may be impregnated with a dispersion including the conductivepolymer doped with a dopant. The dispersion may or may not include theconductive particles. The method for carrying the conductive particleson the separator is not particularly limited. For example, a liquid inwhich the conductive particles are dispersed may be brought into contactwith the separator, and thereafter the separator may be dried.

(Step (iii))

The step (iii) is a step of impregnating the polymer layer formed in thestep (ii) with a non-aqueous solvent. Thus, an electrolyte layerincluding a conductive polymer doped with a dopant, conductiveparticles, and a non-aqueous solvent is formed. The step (iii) may be astep of impregnating the polymer layer formed in the step (ii) with anelectrolytic solution including a non-aqueous solvent. That is, the step(iii) may be a step of impregnating the polymer layer formed in the step(ii) with a liquid component (L).

The impregnation method in the step (iii) is not particularly limited,and a known method may be used. For example, the capacitor elementprecursor that has been subjected to the step (ii) may be immersed inthe non-aqueous solvent (or the electrolytic solution). As thenon-aqueous solvent (or the electrolytic solution) used in step (iii),those described above can be applied.

In the manufacturing method according to the present disclosure, thedopant may be a polymeric dopant containing an acidic group, and thestep (iii) may be a step of impregnating the polymer layer with anelectrolytic solution including a non-aqueous solvent and a basecomponent dissolved in the non-aqueous solvent.

Through the step (iii), a capacitor element is obtained. After the step(iii), an electrolytic capacitor may be produced using the constituentelements obtained in the step (iii). The step used therefor is notparticularly limited, and it is possible to use a known method.

In the following, an exemplary electrolytic capacitor according to thepresent disclosure will be specifically described with reference to thedrawings; however, the electrolytic capacitor according to the presentdisclosure is not limited by the drawings described below. Theabove-described constituent elements can be applied to constituentelements of the exemplary electrolytic capacitor described below. Theconstituent elements of the exemplary electrolytic capacitor describedbelow can be modified based on the above description. The mattersdescribed below may be applied to the above-described embodiment. Thesame portions may be denoted by the same reference numerals, andredundant descriptions may be omitted.

Embodiment 1

In Embodiment 1, an exemplary electrolytic capacitor according to thepresent disclosure will be described. The electrolytic capacitor is anelectrolytic capacitor including a capacitor element. FIG. 1schematically shows a cross section of an exemplary electrolyticcapacitor 100 according to Embodiment 1. FIG. 2 shows a schematicpartial developed view of a capacitor element 10 included in theelectrolytic capacitor 100 shown in FIG. 1 .

As shown in FIG. 1 , the electrolytic capacitor 100 includes thecapacitor element 10, a bottomed case 11 that accommodates the capacitorelement 10, a sealing member 12 that closes an opening of the bottomedcase 11, a seat plate 13 that covers the sealing member 12, lead wires14A and 14B that are led out from the sealing member 12 and penetratethe seat plate 13, and lead tabs 15A and 15B that connect the lead wires14A and 14B to electrodes of the capacitor element 10. The capacitorelement 10 is accommodated in the bottomed case 11. The bottomed case 11is drawn inward at a portion near an opening end thereof, and theopening end of the bottomed case 11 is curled so as to crimp the sealingmember 12.

Referring to FIG. 2 , the capacitor element 10 includes a foil-shapedanode body 21 having a dielectric layer on a surface thereof, afoil-shaped cathode body 22, and a separator 23 and an electrolyte layer(not shown) that are disposed therebetween. The anode body 21 and thecathode body 22 are wound with the separator 23 disposed therebetween.The outermost periphery of the wound body is fixed using winding stoptape 24. Note that FIG. 2 shows the wound body in a partial developmentbefore the outermost periphery of the wound body is fixed.

EXAMPLES

Hereinafter, the embodiment according to the present disclosure will bedescribed in further detail by way of examples.

EXAMPLES Production of Capacitor A1

A capacitor A1 is a wound electrolytic capacitor having a rated voltageof 35 V and a rated capacitance of 270 μF. The capacitor A1 was producedaccording to the following procedure.

Preparation of Cathode Body and Anode Body

As a cathode body, an A1 foil (aluminum foil) having a thickness of 70μm was used. An anode body having a dielectric layer formed on a surfacethereof was produced according to the following procedure. First, an A1foil having a thickness of 120 μm was prepared. The A1 foil wassubjected to direct-current etching treatment, to roughen the surface ofthe A1 foil. Then, the A1 foil was subjected to chemical conversiontreatment. Specifically, the A1 foil was immersed in an aqueous ammoniumadipate solution, and subjected to chemical conversion treatment at 70°C. for 30 minutes, under application of a voltage of 50 V to the A1foil, thereby forming a dielectric layer (thickness: about 70 nm) on asurface of the A1 foil. In this manner, an anode body having adielectric layer formed on a surface thereof was obtained. Thereafter,the anode body was cut into a predetermined size, to prepare an anodebody of the capacitor A1.

Preparation of PEDOT:PSS Dispersion

A dispersion of a conductive polymer doped with a dopant was prepared bythe following method. First, 3,4-ethylenedioxythiophene andpolystyrenesulfonic acid (dopant) were dissolved in ion exchanged water,to prepare a mixed solution thereof. To the resultant mixed solution,iron (III) sulfate (oxidizing agent) dissolved in ion exchanged waterwas added under stirring, to allow polymerization reaction to proceed.After the reaction, the resultant reaction solution was dialyzed toremove the unreacted monomer and excess oxidizing agent. In this manner,a dispersion including poly(3,4-ethylenedioxythiophene) doped withpolystyrenesulfonic acid (about 5 mass % relative topoly(3,4-ethylenedioxythiophene) was obtained. In the following,poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonic acid maybe referred to as “PEDOT:PSS”.

(Addition of Conductive Particles to PEDOT:PSS Dispersion)

To a dispersion including 2 mass % of the above-described PEDOT:PSS,graphene particles (flaky, average maximum diameter D: 0.4 μm) wereadded. In this manner, a treating solution A (dispersion) includingPEDOT:PSS and conductive particles were prepared. The mass ratio betweenthe PEDOT:PSS and the conductive particles was 75:25.

(Separator)

As the separator, a non-woven fabric (average thickness: 40 μm) wasprepared. The non-woven fabric was composed of 50 mass % (25 mass % ofpolyester fibers, 25 mass % of aramid fibers) of synthetic fibers and 50mass % of cellulose, and included polyacrylamide as a paperstrengthening agent.

(Production of Wound Body)

An anode lead tab and a cathode lead tab having the respective leadwires connected thereto were connected to the anode body and the cathodebody, respectively. Then, the anode body and the cathode body were woundwith the separator sandwiched therebetween, and an outer surface wasfixed using a winding stop tape. As the separator, a non-woven fabricmade of cellulose was used. In this manner, a wound body (capacitorelement precursor) was produced. The produced wound body was immersed inan ammonium adipate solution, and subjected to chemical conversiontreatment again at 70° C. for 60 minutes, while applying a voltage of 50V to the anode body. Through this chemical conversion treatment, adielectric layer was formed mainly on an end face of the anode body.

(Formation of Conductive Polymer Layer)

First, the above-described treating solution A was placed in acontainer. Then, at room temperature and under reduced-pressureatmosphere (40 kPa), the wound body was immersed in the treatingsolution A in the container for 15 minutes, and thereafter the woundbody was withdrawn from the treating solution A. In this manner, thewound body is impregnated with the treating solution A. Then, in adrying furnace, the wound body was dried at 60° C. for 30 minutes, andsubsequently at 150° C. for 30 minutes. Thus, the dispersing medium(water) contained in the impregnated treating solution A was removed. Inthis manner, a conductive polymer layer containing the conductiveparticles was formed.

(Impregnation of Electrolytic Solution)

The wound body having the conductive polymer layer formed thereon wasimpregnated with an electrolytic solution at room temperature underatmospheric pressure. As the electrolytic solution, a mixed solution ofpolyethylene glycol, γ-butyrolactone, sulfolane, andmono(ethyldimethylamine)phthalate (solute) at a mass ratio ofpolyethyleneglycol:γ-butyrolactone:sulfolane:mono(ethyldimethylamine)phthalate=30:30:20:20was used. In this manner, a capacitor element including an electrolytelayer was obtained. The capacitor element was sealed to complete anelectrolytic capacitor. Thereafter, the electrolytic capacitor wassubjected to aging treatment at 130° C. for 2 hours, under applicationof a rated voltage. In this manner, a capacitor A1 was obtained.

Production of Capacitor A2

A capacitor A2 was produced using the same materials and conditions asin the case of the capacitor A1 except that the mass ratio betweenPEDOT:PSS and the conductive particles was 25:75.

Production of Capacitor A3

A capacitor A3 was produced using the same materials and conditions asin the case of the capacitor A1 except that nickel particles (spherical,average maximum diameter D: 2 μm) were used as the conductive particlesin the treating solution A.

Production of Capacitor A4

A capacitor A4 was produced using the same materials and conditions asin the case of the capacitor A3 except that the mass ratio betweenPEDOT:PSS and the conductive particles was 25:75.

Production of Capacitor A5

A capacitor A5 produced using the same materials and conditions as inthe case of the capacitor A3 except that nickel particles (spherical,average maximum diameter D: 10 μm) were used as the conductive particlesin the treating solution A, and that the average thickness of theseparator was 30 μm.

Production of Capacitor A6

A capacitor A6 was produced using the same materials and conditions asin the case of the capacitor A5 except that the mass ratio betweenPEDOT:PSS and the conductive particles was 25:75.

Production of Capacitor C1 (Comparative Example)

A capacitor C1 was produced using the same materials and conditions asin the case of the capacitor A1 except that the conductive particleswere not used. Accordingly, the conductive polymer layer of thecapacitor C1 includes PEDOT:PSS, but does not include the conductiveparticles.

(Measurement of ESR)

For the electrolytic capacitors produced as described above, theequivalent series resistance (ESR) was measured. The ESR was measuredunder an environment at 20° C., using an LCR meter for 4-terminalmeasurement. As the ESR, the initial value after production of theelectrolytic capacitor, and the value after the electrolytic capacitorhad been left standing at high temperature (at 145° C. for 150 hours and500 hours) were measured. Then, the relative value of the initial valueof ESR was determined using an equation shown below. In addition, as anindicator of long-term characteristics, the ESR change rate of each ofthe capacitors was determined using an equation shown below. Therelative value of the initial value of ESR and the change rate of ESRare relative values with respect to the initial value of ESR of thecapacitor C1.

Relative value of initial value of ESR (%)=100×(Initial value of ESR ofeach capacitor)/(Initial value of ESR of capacitor C1)

ESR change rate (%)=100×(Value of ESR of each capacitor after leftstanding at high temperature)/(Initial value of ESR of capacitor C1)

(Measurement of Leakage Current)

Under an environment at 20° C., a rated voltage was applied to theelectrolytic capacitor, and a leakage current (LC) was measured after anelapse of 2 minutes. As the leakage current, the initial value afterproduction of the electrolytic capacitor, and the value after theelectrolytic capacitor had been left standing at high temperature (at145° C. for 150 hours and 500 hours) were measured. Then, the relativevalue of the initial value of the leakage current was determined usingan equation shown below. In addition, as an indicator of long-termcharacteristics, the leakage current change rate was determined using anequation shown below. The initial value of the leakage current and thechange rate of the leakage current were relative values with respect tothe initial value of the leakage current of the capacitor C1.

Relative value of initial value of leakage current (LC) (%)=100×(Initialvalue of leakage current of each capacitor)/(Initial value of leakagecurrent of capacitor C1)

Leakage current change rate (%)=100×(Value of leakage current of eachcapacitor after left standing at high temperature)/(Initial value ofleakage current of capacitor C1)

Some of the conditions for forming the electrolyte layer of theelectrolytic capacitor described above are shown in Table 1. Note thatthe mass ratio (or content ratio) between the conductive particles andPEDOT:PSS in the formed electrolyte layer can be regarded as being equalto the ratio between their contents in the treating solution.Accordingly, the mass ratio between the conductive particles andPEDOT:PSS in the electrolyte layer was calculated from their contents inthe treating solution. In addition, evaluation results of the ESR andleakage current for the electrolytic capacitors described above areshown in Table 2. It is preferable that the ESR and the leakage currentare low.

TABLE 1 Mass ratio in Average electrolyte maximum Separator Conductiveparticles layer diameter Average Average (PEDOT· D/ thickness maximumPSS): Average T diameter D Conductive thickness (μm) Material (μm)particles T A1 40 Graphene  0.4 75:25 0.01 A2 40 Graphene  0.4 25:750.01 A3 40 Nickel  2 75:25 0.05 A4 40 Nickel  2 25:75 0.05 A5 30 Nickel10 75:25 0.33 A6 30 Nickel 10 25:75 0.33 C1 40 No conductive particles

TABLE 2 Relative ESR change Relative LC change value of rate [%] valueof rate [%] initial value After After initial value After After of ESR[%] 150 hr 500 hr of LC [%] 150 hr 500 hr A1 52 63 74 119 136.9 160.7 A222 25 27 135 155.3 182.3 A3 46 79 110 124 142.6 167.4 A4 56 69 86 128147.2 172.8 A5 60 103 143 107 123.1 144.5 A6 73 89 112 118 135.7 159.3C1 100 119 170 100 114.0 132.0

As shown in Table 2, the ESRs of the capacitors A1 to A6 weresignificantly lower than the ESR of the capacitor C1 of a comparativeexample.

In general, adding conductive particles to a conductive polymer layerleads to an increased leakage current. However, the leakage current wassuccessfully reduced by increasing the value of (Average maximumdiameter D/Average thickness T). In addition, the effect of reducing theleakage current by increasing the value of (Average maximum diameterD/Average thickness T) was significant when the amount of the conductiveparticles was larger than the amount of the conductive polymer.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to an electrolytic capacitor and amanufacturing method thereof.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

REFERENCE SIGNS LIST

-   -   10. . . . Capacitor element    -   21. . . . Anode body    -   22. . . . Cathode body    -   23. . . . Separator    -   100. . . . Electrolytic capacitor

1. An electrolytic capacitor comprising a capacitor element, wherein thecapacitor element includes an anode body, a dielectric layer formed on asurface of the anode body, a cathode body, and an electrolyte layer anda separator that are disposed between the dielectric layer and thecathode body, the electrolyte layer includes a non-aqueous solvent andconductive particles, and a ratio D/T of an average maximum diameter Dof the conductive particles to an average thickness T of the separatoris in a range of 0.01 to 0.9.
 2. The electrolytic capacitor according toclaim 1, wherein the electrolyte layer further includes a conductivepolymer.
 3. The electrolytic capacitor according to claim 2, wherein theelectrolyte layer includes a dopant of the conductive polymer.
 4. Theelectrolytic capacitor according to claim 3, wherein the dopant is apolymeric dopant containing an acidic group, and the electrolyte layerincludes an electrolytic solution including the non-aqueous solvent anda base component dissolved in the non-aqueous solvent.
 5. Theelectrolytic capacitor according to claim 4, wherein a content of thebase component in the electrolytic solution is 0.1 mass % or more and 20mass % or less.
 6. The electrolytic capacitor according to claim 3,wherein a mass of the conductive particles included in the electrolytelayer is larger than a total mass of the conductive polymer and thedopant included in the electrolyte layer.
 7. The electrolytic capacitoraccording to claim 1, wherein the conductive particles are particles ofa conductive carbon material.
 8. The electrolytic capacitor according toclaim 1, wherein the cathode body is a conductive foil, and theconductive particles are metal particles.